[0001] This invention relates to a method and a cell for metal production by electrolysis
of a molten electrolyte which is more dense than the metal. The invention will be
particularly described with reference to the production of magnesium by electrolysis
of a molten electrolyte containing magnesium chloride. But it should be understood
that the invention is also applicable to other electrolytes and other metals.
[0002] In-the electrolysis of molten electrolytes containing magnesium chloride, magnesium
is formed at the cathode and chlorine at the anode. Since both are lighter than the
electrolyte, both migrate to the surface. If the magnesium and the chlorine come into
contact with one another, they tend to re-combine, and this is a major cause of production
losses. The tendency is a function of the contact time, the intimacy of contact and
the electrolyte temperature.
[0003] The classical solution to this problem was to separate anode and cathoderegions by
means of a diaphragm. But a diaphragm considerably increases the interelectrode distance
and therefore the internal resistance of the cell and although this solution has been
used commercially for many years, the more recent industrial practice has favoured
diaphragmless cells. Cells without diaphragms may be divided into two categories:-
i) those cells designed to keep the magnesium generated at the cathode essentially
free from contact with the chlorine generated at the anode. To do this, it is necessary
to keep a substantial distance between facing electrodes, and this in turn means that
a substantial amount of electrical energy must be spent overcoming the electrical
resistance of the electrolyte.
[0004] Such cells have high current efficiency because magnesium/chlorine recombination
is substantially prevented. ii) those cells designed to use the chlorine to lift the
magnesium droplets to the surface of the electrolyte. The anode/cathode spacing can
be greatly reduced, thus reducing the internal resistance of the cell, but the current
efficiency is lowered by reason of back reaction of Mg and C1
2. The current-efficiency of the cell is dependent upon the rapidity of separation
of the product Mg from the generated chlorine. The cells of this invention are in
category (ii).
[0005] One of the cells of category i) is described in U.S. Patent 4,055,474 by this inventor.
In this cell use is made of inverted steel troughs extending above each cathode and
beneath the surface of the bath to receive the rising metal and conducting it to a
suitable metal collection locality separated from the main chlorine collecting chamber.
The electrolyte circulation is obtained by the gas lift effect in the interelectrode
space. After release of the chlorine above the steel troughs the electrolyte flows
downwards in spaces provided on the back of the cathodefaces.
[0006] The same product separating technique has been recently proposed (European Patent
Specification 27016A) for a cell provided with intermediate bipolar electrodes where
inverted troughs are designed on the cathodic surfaces for the individual collection
of magnesium metal and delivery outwards to a separate reservoir. A
243 similar arrangement is suggested for the collection of chlorine on the anodic surfaces.
The interelectrode spacings and the inclination of the electrode surfaces, especially
the cathodic surfaces, are selected to satisfactorily separate the two products. Experience
has shown that a minimum spacing of 5cm is necessary to prevent mixing and therefore
a substantial voltage drop results, even when the electrode geometry is optimized,
from the passage of current at the densities required to produce commercial quantities
of magnesium.
[0007] A cell in category ii) is described in U.S. Patent 3907651, in which there are used
assemblies of double-acting anodes and double-acting cathodes, the latter each having
a passage between the two anode-facing surfaces through which an electroiyte/magnesium
mixture passes to a separate metal collection chamber. A restriction may be provided
at the entrance to this passage to assist in the separation of chlorine from the liquid
mixture. The design suffers from the difficulty of designing the passage so that the
flow of electrolyte is sufficiently fast to maintain magnesium droplets in suspension
but sufficiently slow to permit complete de-gassing.
[0008] Multipolar cells of category ii) have been proposed (U.S. Patents 2,4.68,022 and
2,629,688) where the collection of magnesium is effected by circulating the electrolyte
towards a metal collecting locality by means of a mechanical pump: the interelectrode
spaces between bipolar vertical slabs are swept by the circulating electrolyte and
the magnesium produced is made to overflow into a common sump disposed alongside the
spaces and separated from them by submerged weirs which prevent the passage of chlorine
from the electrolysis chamber and the sump. The metal is retained by a dam disposed
in the metal collecting chamber, so that only electrolyte is pumped back into the
electrolysis chamber. The operating difficulties arising from the need to maintain
the pump in continuous use in spite of the difficult environment are well known to
those skilled in the art. This may be the reason why these cells have not been very
successful commercially.
[0009] We have now found a method to effect the separation of magnesium in cells of multipolar
design by means of circulating electrolyte without the use of pumps. The electrolyte
circulation is obtained by using small interelectrode spaces and a high current density
at the electrodes which leads to a high rate of lift of electrolyte (because of the
high rate of chlorine flow in the interelectrode spaces) without however any excessive
voltage drop (because of the small interelectrode distance) and to a satisfactory
current efficiency (because of the very rapid separation of the products).
[0010] In our copending patent application No. 83303025.7 '(filed on the 25th may, 1983.
) the electrolyte circulation is made to occur sideways in the planes of the interelectrode
spaces. In that mode of circulation the time required for the electrolyte/metal mixture
to reach the side discharge point increases with the increasing width of the electrodes,
so that a limit is reached for the optimum electrode width beyond which the -current
efficiency of the cell becomes less advantageous
[0011] We have now found a method to overcome this problem and still retain all the other
advantages described in the copending patent application.
[0012] The present invention provides in one aspect an electrolytic cell for the production
of a metal by electrolysis of a molten electrolyte which is more dense than the metal,
comprising,
an electrolysis chamber including at least one electrode assembly of an anode, one
or more intermediate bipolar electrodes, and a cathode having a front, face facing
an intermediate bipolar electrode and a back face, the electrodes defining electrolysis
regions between them, and a gas collection space above the assembly,
a metal collection chamber in communication with the top and bottom of the electrolysis
regions, but screened from the gas collection space,
a duct extending adjacent the back face of the cathode and leading to the metal collection
chamber, including a restricted passage for electrolyte/metal mixture and, downstream
of the restricted passage, an inverted channel for metal collection contoured to cause
metal to flow to the metal collection chamber,
the one or more intermediate bipolar electrodes having top edges arranged to permit
electrolyte/metal mixture rising from the electrolysis regions to spill out over the
cathode and into the duct,
and means for maintaining the surface of the electrolyte/metal mixture at a substantially
constant level.
[0013] The present invention provides in another aspect a process for the production of
a metal by electrolysis of a molten metal chloride electrolyte which is more dense
than the metal, which method comprises,
introducing electrolyte into the lower ends of interelectrode regions between the
electrodes of one or more assemblies each comprising an anode, a cathode and one or
more intermediate bipolar electrodes,
passing an electric current between the anode and the cathode whereby chlorine is
generated at anodic electrode faces, the metal is generated at cathodic electrode
faces, and an electrolyte/metal/chlorine mixture is caused to rise up the interelectrode
regions,
causing the elctrolyte/metal mixture which emerges from the upper ends of the interelectrode
regions to spill over the or each intermediate bipolar electrode and over the cathode
and to pass down a restricted passage behind the cathode,
maintaining the liquid surface level at a substantially constant height to effect
substantially complete separation of chlorine from the electrolyte metal mixture at
or upstream of the restricted passage without permitting a significant proportion
of electric current to by-pass the intermediate electrode(s), and
downstream of the restricted passage, separating and recovering metal from electrolyte/metal
mixture into an inverted channel leading to a metal collection region and recirculating
electrolyte to the lower ends of the interelectrode regions.
[0014] Intermediate bipolar electrodes used in this invention are valuable in that they
increase the effective cathode area on which metal formation can take place, without
either increasing the size of the cell or increasing the heat and power loss involved
in providing large numbers of external electrical connections. One problem which intermediate
bipolar electrodes generate is that of current leakage. Because the polarization voltage
arising from the electrolysis process in each interelectrode space is quite high,
current tends to flow where possible through the electrolyte/metal mixture and round,
rather than through, the intermediate bipolar electrodes. This invention provides
several features designed to mitigate this problem:-
. a) Current leakage over the top of the intermediate bipolar electrodes can be minimised
by operating a level control device to keep the liquid surface at about the level
of the top edges of these electrodes. Thus, the liquid surface should preferably be
no higher than is necessary to permit the electrolyte/metal mixture rising from the
electrolysis regions to spill out over the cathode and into the duct.
b) Current leakage round the ends of the intermediate bipolar electrodes can be substantially
avoided by providing electrical insulation, e.g. refractory blocks adjacent each end
of the electrode assembly. But such blocks are inevitably worn away or cracked during
prolonged operation, leading to a gradual increase in by-pass currents.
c) Current leakage below the bottom edges of the intermediate bipolar electrodes cannot
be entirely eliminated because of the need to provide passages for the entry of electrolyte
to the lower ends of the electrolysis regions. Current leakage here can be minimised
by restricting the size of the passages and/or by providing a tortuous flow path for
the electrolyte (and the electric current).
d) In a preferred embodiment of this invention, intermediate bipolar electrodes and
cathodes are arranged, not only facing the major faces of the anode, but also facing
the ends and/or the bottom faces of the anode. By this means, each anode can be completely
surrounded by intermediate bipolar electrodes. The design nests completely the high
voltage zone surrounding the anode, and provides a very functional electrode configuration
which allows the use of a relatively large number of poles in the cell without suffering
significantly from the problem of current by-pass and refractory wear.
[0015] In operation, a mixture of electrolyte, molten metal and gas, typically chlorine,
streams upwards through the electrolysis regions. The electrolyte/metal mixture spills
over the or each intermediate bipolar electrode, over the cathode and into the duct
behind the cathode. For this to be possible, it is necessary that the top edge of
the intermediate bipolar electrode adjacent the front face of the cathode be at least
as high as the top edge of the cathode. If there are more than one intermediate electrodes,
no intermediate electrode should be significantly higher than one between it and the
anode. Preferably, the tops of all the intermediate bipolar electrodes (when more
than one is used) are substantially at the same height or are located on a slight
incline going up from cathodeto anode. To provide a uniform flow of electrolyte/metal
mixture over them, the top edges of the intermediate bipolar electrode(s) and the
cathode should be essentially horizontal along their length.
[0016] The duct extending adjacent the back face of the cathode includes a restricted passage
for electrolyte/ metal mixture, preferably at substantially the level of the top edge
of the cathode. This restricted passage serves to control the flow of the mixture
so as to provide a pressure drop which prevents metal droplets from returning countercurrent
through it, this pressure differential being sufficient to prevent metal collected
in the inverted channel and in the metal collection chamber from returning to the
electrolysis chamber if a leak develops. Therefore, efficient collection of metal
will be retained for-a long time until cell damage is extensive.
[0017] The restricted passage may be constituted by baffles that function as gas deflectors
and separators at the entrance to the duct. The design of these deflectors may follow
conventional hydrodynamic principles. If the liquid surface level is too high, a significant
proportion of electric current may by-pass the intermediate electrode(s) and also
molten metal may coalesce in the electrolysis chamber, floating in the gas collection
space rather than being entrained in the circulating electrolyte. If the level is
too low, chlorine or other gas may be carried over into the metal collection chamber.
Preferably, the surface is maintained at substantially the level of the top edges
of the intermediate bipolar electrode(s). A level control device may be provided to
maintain the liquid surface level substantially constant. This device may take the
form of a vessel, partly or wholly submerged in the electrolyte of the metal collection
chamber, to or from which electrolyte can be transferred to alter the surface level.
Alternatively, the liquid surface level can be maintained substantially constant by
continuous or frequently intermittent tapping of molten metal and/or introduction
of fresh raw material.
[0018] The number of intermediate bipolar electrodes per electrode assembly is not critical,
and may conveniently be from 1 to 7. The electrodes may be arranged vertically or
at a small angle to the vertical. Cathodes or intermediate bipolar electrodes which
face the bottom of an anode may need to be set at an angle or even horizontal, but
the extent of such electrodes should preferably be limited. The cell may include a
single electrode assembly. Alternatively, the cell may include several, e.g. 3 to
8, electrode assemblies, with double-acting cathodes between assemblies. The double-acting
cathodes may include two metal plates constituting the cathodes with between them
a duct leading to the metal collection chamber.
[0019] The cells of this invention are designed to operate at temperatures only slightly
above the melting point of the metal being produced, so as to minimise back-reaction
between the metal and chlorine. When used to produce magnesium (M.P. 651
0) the cell is preferably operated at 655°C-695°C. particularly 660°C to 670°C.
[0020] The cells of this invention are designed to be operated at high current densities,
typically from 0.3
A/cm
2 to 1.5 A/cm
2, and small interelectrode spacings, typically 4mm to 25mm. The anodes and intermediate
bipolar electrodes are preferably of graphite, but may be a composite with a graphite
anodic face and a steel cathodic face. Under these conditions, electrode dimensions
are rather critical to cell efficiency, so all normal precautions must be taken to
prevent entry of air or moisture into the electrolysis chamber so as to reduce consumption
of the graphite anodes and intermediate electrodes. Usually, the gas collection space
in the electrolysis chamber is contained within a closure through which the anodes
project. Preferably, there is provided also a single secondary hood surrounding the
anodes, or a secondary hood surrounding each anode. The space(s) between the closure
and the secondary hood(s) may be filled with inert gas.
[0021] The metal collection chamber may be sealed according to the method described in European
Patent Specification 60048 A.
[0022] Reference is directed to the accompanying drawings, in which:-
Figure 1 is a front elevation of an electrolytic cell according to the invention,
sectioned at two planes (marked A and A in Figure 2);
Figure 2 is a sectional side elevation along the line B - B of Figure 1;
Figure 3 is a plan view, partly in section, of an alternative design of electrolytic
cell according to the invention;
Figure 4 is a sectional end elevation taken along the line C - C of Figure 3; and
Figure 5 is a sectional side elevation taken along the line D - D of Figure 3.
[0023] Referring to Figures 1 and 2, the electrolytic cell comprises a steel outer shell
10, and layer 12 of thermal insulation, and a massive refractory lining 14 of material
which is resistant to both molten magnesium (when the cell is designed to produce
magnesium) and the molten electrolyte to be used. The cell includes an electrolysis
chamber 16, a magnesium collection chamber 18, a duct 20 leading from the top of the
electrolysis chamber 16 to the metal collection chamber 18 and a level control device
22 positioned in the metal collection chamber.
[0024] The electrolysis chamber 16 comprises three electrode assemblies, each including
an anode 24, two cathodes 26, and four pairs of intermediate bipolar electrodes 28,
30, 32, 34. The electrodes are spaced from one another by means of insulating spacers
(not shown), and are arranged vertically so as to provide vertical interelectrode
spaces between adjacent electrodes.
[0025] The cathodes 26 rest on the refractory floor 14 of the cell. Between the pair of
cathodes bounding each electrode assembly, bridges of refractory blocks 36 support
rows of longitudinal refractory blocks 38, on each of which rests an anode or an intermediate
electrode. The blocks 38 are of graded heights, the highest supporting the anode 24
and the lowest supporting the intermediate bipolar electrode 34 adjacent the cathode
26. In this way a configuration for fast electrolyte flow across the tops of the bipolar
electrodes is achieved while nevertheless using bipolar electrodes of constant size.
[0026] The electrolysis chamber is lined, at the bottom by the longitudinal blocks 38, at
the back and sides by the refractory lining 14 of the cell, and at the front by a
curtain wall 40 of refractory blocks. This curtain wall 40 has downward extensions
at 42 which rest on the bridges 36 and separate the electrode assemblies from the
metal collection chamber 18. Between electrode assemblies, the curtain wall 40 extends
down only far enough to separate magnesium metal in the collection chamber 18 from
a head space 44 in the electrolysis chamber. Chlorine gas is retained in this head
space by the roof 46 of the cell, and removed therefrom by a pipe 48.
[0027] Each anode 24 projects through the roof 46 of the cell and is connected to an anode
bus bar 50. A potential problem is diffusion of gas from the atmosphere through the
anodes (which are to some extent porous) into the electrolysis chamber. This problem
is avoided by providing a secondary hood 52 round the top of each anode, and by ensuring
that the region within this secondary hood is either filled with an inert gas such
as argon or maintained at a pressure not greater than the pressure in the head space
44. Alternatively, a single removable hood could be provided round the tops of all
the anodes. The cathodes 26 are connected, through the side wall of the cell, to a
cathode bus bar 54. Connections are positioned well below the bottom of the other
electrodes, so that corrosion of the refractory blocks 14 of the back wall is minimised
in the electrolysis region.
[0028] The tops of the four intermediate bipolar electrodes 28, 30, 32, 34 are all at substantially
the same height, with the top of 28 being slightly higher than 30, which is slightly
higher than 32, which in turn is slightly higher than the top of 34. The top of each
is rounded at 56 on its anode-facing side, to provide as far as possible a smooth
non-turbulent path for electrolyte/ metal mixture rising from the interelectrode regions
to the duct 20. The top of the cathode 26 is lower than "the tops of the intermediate
bipolar electrodes, and the cathode is designed to remain submerged throughout operation.
[0029] A restricted passage 58 is provided in the duct 20 adjacent the top of the cathodes.
Fixed to the back of each cathode is a row of refractory blocks 60. The restricted
passage lies between facing pairs of these refractory blocks, or, at the ends of the
electrolysis chamber, between a refractory block 60 and the wall 14 of the cell. Inverted
channels 62 for metal collection , are mounted on the back of each cathode 26 immediately
below the refractory blocks 60. If desired, these channels 62 may be arranged to slope
gently upwards from the back of the cell towards the metal collection chamber 18 to
which they lead.
[0030] In the metal collection chamber, magnesium metal settles out as a surface layer 64
above an interface 66, the lower part of the chamber being filled with electrolyte.
A metal tap hole 68 is provided.
[0031] The level control device 22 comprises a horizontal jacketed cylindrical vessel 70
closed at both ends and submerged in the electrolyte. The vessel is supported at both
ends by pipes 72 which conduct air into and out of the jacket 74 as necessary to serve
as a heat exchanger. The air inlet pipe is insulated at 76 to avoid local freezing
of metal (as described in European Patent Specification 60048 A). A small diameter
pipe (not shown) enables argon to be fed into, or out of the upper part 78 of the
interior of the vessel. In the lower part of the vessel are holes 80 for the entry
and exit of electrolyte. The surface of the electrolyte/magnesium mixture in the collection
chamber can be raised by feeding argon into the vessel 70, thus expelling electrolyte,
and can be lowered by bleeding argon out of the vessel. Automatic sensing means (not
shown) can be provided to detect the surface level and maintain it substantially constant,
e.g. during tapping of the magnesium or during introduction of magnesium chloride
or other electrolyte components.
[0032] In operation, an electric current is passed between the anodes 24 and the cathodes
26 in the electrolysis chamber. The electrolyte is a conventional mixture of alkali
and alkaline earth metal chlorides and possibly also fluorides, including magnesium
fluoride, designed to be liquid at the chosen operating temperature just above the
melting point of magnesium metal. Molten magnesium is formed on the cathodes 26 and
on the anode-facing surfaces of the intermediate bipolar electrodes 28, 30, 32 and
34. Chlorine is formed on the anodes 24 and on the cathode-facing surfaces of the
intermediate bipolar electrodes. A stream of rising chlorine bubbles fills the interelectrode
space and the resulting upward flow of electrolyte entrains droplets of molten magnesium.
The electrolyte/magnesium mixture reaching the liquid surface at the top of the electrolysis
regions spills over the intervening intermediate electrodes and the cathode towards
the duct 20. The electrolyte/metal mixture then passes down through the restricted
passage 58, designed to produce a liquid flow of controlled' turbulence to entrain
magnesium droplets in the electrolyte and located at such a depth from the electrolyte
surface as to cause any remaining chlorine gas to be released before the electrolyte/metal
mixture reaches the passage. The dimensions of the restricted passage 58 are preferably
such that there is a pressure drop across the passage of from 5 to 50 mm.
[0033] A key feature of the invention is the control of the surface level, in relation both
to the tops of the intermediate bipolar electrodes and to the restricted passage.
Asnoted above, the liquid surface should not be significantly higher than the tops
of the intermediate bipolar electrodes, so as to minimise electric by-pass currents.
The position of the restricted passage in relation to the liquid surface is a compromise
between the need to achieve complete chlorine separation and the need to avoid a quiescent
surface layer where magnesium droplets may coalesce and re-combine with chlorine.
[0034] Below the restricted passage 58, the flow of electrolyte slows down and turns through
90° towards and into the metal collection chamber 18. From there, the electrolyte
turns through 180° and flows back below the electrode assemblies. Then the flow turns
upward, between the insulating blocks 38, and into and up the electrolysis regions
between the electrodes. Most of the magnesium metal entrained in the electrolyte passing
through the restricted passage 58 is released in the duct 20 and collects in the inverted
channel 62. Further magnesium metal is released by the electrolyte in the collection
chamber 18. Magnesium from both these sources floats to the surface in the collection
chamber 18 from where it is tapped.
[0035] Figures 3, 4 and 5 show an alternative design of electrolytic cell. Referring to
these drawings, the cell comprises an electrolysis chamber 100, a metal collection
chamber 102, a duct including a restricted passage 104 for electrolyte/metal mixture
and an inverted channel 106 for metal collection, and a level control device 108 positioned
in the collection chamber.
[0036] The electrolysis chamber contains a single anode 110 in the form of elongated wedge
shaped blocks of graphite positioned next to each other along a continuous axial line,
and connected to an electrical supply by means of an anode bus bar 112. The anode
is completely surrounded by steel cathodes 114 connected to an electrical supply by
a cathode bus bar 116. The cathodes comprise side faces, 118 at a small angle to the
vertical and facing the major faces 119 of the anode; and vertical end faces '120
facing the vertical ends 121 of the anode. Sandwiched between the cathode faces 118
and the anode faces 119 are four intermediate bipolar electrodes 122. Sandwiched between
the cathode faces 120 and the anode ends 121 are four intermediate bipolar electrodes
124. Steel plates 126 are welded to the faces 118 of the cathodes towards their bottom
edge. These plates, which form extensions of the cathodes, are inclined at an angle
of about 45° to the vertical. Between these plates 126 and the bottom 128 of the anode
are positioned three intermediate bipolar electrodes 130, also inclinded at about
45° to the vertical. A narrow gap 132 is left between the inclined sets of intermediate
electrodes 130 for entry of electrolyte into the system. The inclined electrolysis
regions between the plates 126 and the intermediate electrodes 130 are in communication
with the substantially vertical electrolysis regions between the cathode faces 118,
the intermediate electrodes 122 and the anode 110, so that there is a continuous flow
of electrolyte up these regions. All electrodes are spaced from one another by means
of insulating spacers (not shown).
[0037] The cell comprises a steel outer shell 134, a layer 136 of thermal insulation,- and
a massive refractory lining 138 of material which is resistant to both molten magnesium
and the molten electrolyte to be used. The electrolysis region is closed by means
of an insulated lid 140 provided with a vent 142 for removal of chlorine gas.
[0038] The magnesium collection chamber 102 is separated from the electrolysis chamber 100
by means of a curtain wall 144 which extends down from the roof of the cell to below
the electrolyte surface, supported by pillars 145. In the collection chamber, magnesium
metal rises to the surface and forms a layer 146 above an interface 148, from which
it can be removed by means not shown. A level control device 108 is similar to that
described and illustrated in Figures 1 and 2 and consists of an elongated horizontal
vessel 152 supported at both ends by pipes 153, with holes 154 on its bottom side
for entry or exit of electrolyte. Means (not shown) are provided for controlling the
flow of argon gas into or out of the upper part of this vessel, so as to draw in,
or expel, electrolyte from the vessel and change the surface level in the cell accordingly.
[0039] Adjacent the back faces of the cathode, 118, 120 are blocks 156 of insulating material.
On three sides of the electrolysis chamber, the restricted passage 104 for electrolyte/metal
mixture is formed between these blocks and the insulating blocks 138 lining the cell.
On the fourth side, between the electrolysis chamber and the magnesium collection
chamber, the restricted passage 104 is formed between the insulating blocks 156 and
the curtain wall 144. Mounted below the blocks is the inverted channel 106 for the
collection of magnesium metal. This channel extends continuously all round the electrode
assembly, and extensions 158 are provided to convey metal below the curtain wall 144
into the magnesium collection chamber. The channel may, but need not, slope upwards
towards the magnesium collection chamber.
[0040] The sloping metal plates 126 form, with the bottom edges of the electrode faces 118,
secondary channels 160 for magnesium collection. Apertures 162 in the bottom edges'of
the electrode faces 118 permit passage of . magnesium metal from these secondary channels
and up to the primary collection channels 106.
[0041] The steel cathodes are divided by expansion joints 164 into elements small enough
for the different rates of thermal expansion of steel and graphite not to become a
serious problem. The expansion joints are of such a size as to avoid the accumulation
of movement as the number of cathodes increases.
[0042] Operation of the cell is similar to that of the cell described in Figures 1 and 2.
A mixture of electrolyte,. magnesium and chlorine streams up the electrolysis regions
between the electrodes, and spills over the intermediate electrodes and the cathode
onto the refractory blocks 156 and down the restricted passages 104..Thereafter, the
rate of electrolyte flow slows down, the magnesium droplets are collected in the channels
106 and 160 and passed to the magnesium collection chamber. Freed of magnesium metal,
the electrolyte enters the passage 132, and so passes up again into the electrolysis
regions between the electrodes. Thus, electrolyte substantially circulates round the
cathodes, and circulation of electrolyte to and from the magnesium collecting chamber
is only partial.
[0043] By virtue of the cathodes 120 and 126, and intermediate electrodes, 124 and 130,
surrounding the ends 121 and the bottom 128 of the anode, electrical by-pass currents
are reduced to a very low level.. Thus, the cell achieves the advantages of using
intermediate bipolar electrodes, that they increase the effective cathode area on
which metal formation can take place, without either increasing the size of the cell
or .increasing the heat and power loss involved in providing large numbers of external
electrical connections, while avoiding a major potential disadvantage of such intermediate
bipolar electrodes.
[0044] The cell described illustrated in Figures 3 to 5 represents our currently preferred
embodiment, but could be modified in various ways within the scope of the invention:-
a) The cathode faces 126 and the intermediate bipolar electrodes 130 could be omitted
and replaced by insulating blocks designed to minimise electrical by-pass currents
below the intermediate electrodes 122 and 124.
b) The cathode faces 120 and the intermediate bipolar electrodes 124 could be omitted
and replaced by insulating blocks designed to minimise electrical by-pass currents
round the ends of the intermediate electrodes 122 and 130.
c) Both steps a) and b) could be taken at the same time, leaving only the intermediate
electrodes 122.
d) In place of a single anode, several rectangular anodes could be used, each surrounded
on some or all sides by cathodes and intermediate bipolar electrodes.
e) The rectangular anode(s) could be arranged to extend perpendicular to, rather than
parallel to, the adjoining magnesium collection chamber.
f) The anode(s) could have a horizontal cross- section which is square or circular
rather than rectangular.
g) The anode(s) could taper in a downward direction, i.e. the anode(s) could be conical
or pyramidal, rather than cylindrical or rectangular.
1. An electrolytic cell for the production of a metal by electrolysis of a molten
electrolyte which is more dense than the metal, comprising,
an electrolysis chamber 16 including at least one electrode assembly of an anode 24,
one or more intermediate bipolar electrodes 28, 30, 32, 34 and a cathode 26 having
a front face facing an intermediate bipolar electrode and a back face, the electrodes
defining electrolysis regions between them, and a gas collection space 44 above the
assembly,
a metal collection chamber 18 in communication with the top and bottom of the electrolysis
regions, but screened from the gas collection space,
a duct 20 extending adjacent the back face of the cathode and leading to the metal
collection chamber, including a restricted passage 58 for electrolyte/metal mixture
and, downstream of the restricted passage, an inverted channel 62 for metal collection
contoured to cause metal to flow to the metal collection chamber,
the one or more intermediate bipolar electrodes having top edges arranged to permit
electrolyte/metal mixture rising from the electrolysis regions to spill out over the
cathode and into the duct,
and means 22, for maintaining the surface of the electrolyte/metal mixture at a substantially
constant level.
2. A cell as claimed in claim 1, wherein the anode has a major face and also an end
face and/or a bottom face, and one or more intermediate bipolar electrodes are arranged,not
only facing the major face of the anode, but also facing the end face and/or the bottom
face of the anode.
3. A cell as claimed in claim 1 or claim 2, wherein the top edge of each intermediate
bipolar . electrode is horizontal and rounded on its anode-facing side.
4. A cell as claimed in any one of claims 1 to 3, wherein the top edges of all the
intermediate bipolar electrodes are substantially at the same height.
5. A cell as claimed in any one of claims 1 to 4, wherein the means for maintaining
the surface of the electrolyte/metal mixture at a substantially constant level comprises
a level control device in the form of a vessel partly or wholly submerged in the electrolyte
of the metal collection chamber, to or from which electrolyte can be transferred to
alter the surface level.
6. A process for the production of a metal by electrolysis of a molten metal chloride
electrolyte which is more dense than the metal, which method comprises,
introducing electrolyte into the lower ends of interelectrode regions between the
electrodes of one or more assemblies each comprising an anode, a cathode and one or
more intermediate bipolar electrodes,
passing an electric current between the anode and the cathode whereby chlorine is
generated at anodic electrode faces, the metal is generated at cathodic electrode
faces, and an electrolyte/metal/chlorine mixture is caused to rise up the interelectrode
regions,
causing the electrolyte/metal mixture which emerges from the upper ends of the interelectrode
regions to spill over the or each intermediate bipolar electrode and over the cathode
and to pass down a restricted passage behind the cathode,
maintaining the liquid surface level at a substantially constant height to effect
substantially complete separation of chlorine from the electrolyte metal mixture at
or upstream of the restricted passage without permitting a significant proportion
of electric current to by-pass the intermediate electrode(s), and
downstream of the restricted passage, separating and recovering metal from electrolyte/metal
mixture into an inverted channel leading to a metal collection region and recirculating
electrolyte to the lower ends of the interelectrode regions.
7. A process as claimed in claim 6, wherein the liquid surface is maintained at about
the level of the top edges of the intermediate bipolar electrodes.
8. A process as claimed in claim 6 or claim 7, wherein the electrolyte/metal mixture
undergoes a pressure drop of from 5 - 50 mm upon passing through the restricted passage.
9. A process as claimed in any one of claims 6 to 8, wherein a molten electrolyte
comprising magnesium chloride is used to produce magnesium metal.
10. A method as claimed in claim 9, wherein the cell is operated at a temperature
of from 6550C to 6950C, a current density of from 0.3 A/cm2 to 1.5 A/cm2 and interelectrode spacings of from 4 mm to 25 mm.